Glycoprotein gp85, the product of the BXLF2 open reading frame (ORF), is the gH homolog of Epstein-Barr virus (EBV) and has been implicated in penetration of virus into B cells. Like its counterparts in other herpesviruses, it associates with a gL homolog, gp25, which is the product of the BKRF2 ORF. Unlike the gH homologs of other herpesviruses, however, gp85 also complexes with two additional glycoproteins of 42 and 38 kDa. Glycoproteins gp42 and gp38 were determined to be alternatively processed forms of the BZLF2 gene product. Coexpression of EBV gH and gL facilitated transport of gH to the cell surface and resulted in formation of a stable complex of gH and gL. It also restored expression of an epitope recognized by monoclonal antibody E1D1, which immunoprecipitates the native gH complex but not recombinant gH expressed in isolation. Coexpression of gH, gL, and the BZLF2 ORF restored expression of an epitope recognized by a second monoclonal antibody, F-2-1, which immunoprecipitates the native gH-gL-gp42/38 complex but not the complex of recombinant gH and gL alone. The epitope recognized by antibody F-2-1 was mapped to the BZLF2 gene product itself. Antibody F-2-1 inhibited the ability of EBV to infect B lymphocytes but had no effect on the ability of the virus to infect the epithelial cell line SVK-CR2. In contrast, antibody E1D1 had no effect on infection of the B-cell line but inhibited infection of the epithelial cell line. These results indicate that penetration of the two cell types by EBV involves differential use of the gH-gL-gp42/38 complex and suggest the hypothesis that the BZLF2 gene product has evolved as a unique adaptation to infection of B lymphocytes by EBV.

We described previously a preleukemic state in mice inoculated with Moloney murine leukemia virus (M-MuLV) characterized by generalized hematopoietic hyperplasia in the spleen. To investigate this further, long-term bone marrow cultures (LTBMC) from preleukemic mice were established. Surprisingly, LTBMC from M-MuLV-inoculated preleukemic mice showed less hematopoiesis than LTBMC from control mice. This resulted from a quantitative defect in establishment of bone marrow stromal cells in the LTBMC. This phenomenon could also be observed in LTBMC from normal mice infected in vitro with a stock of M-MuLV containing a mink cell focus-forming virus (MCF) derivative (M-MCF), but not in LTBMC infected with M-MuLV alone. This implicated MCF derivatives in the reduction in bone marrow stromal cells. The phenomenon could also be detected in infected NIH 3T3 cells. Combined infection of M-MuLV plus M-MCF resulted in fewer cells, in comparison to uninfected cells or cells infected with either virus alone. Further studies indicated that this was predominantly due to an inhibition in cell growth rather than to cell lysis. The cytopathic effect did not appear to result from overreplication of viral DNA, as measured by Southern blots. Thus, combined infection with M-MuLV and an MCF derivative had cytostatic effects on cell growth. This phenomenon might also contribute to the leukemogenic process in vivo.